Loved the demo! I think you got the retro style very well, everything from the art, music, sound fx, gameplay.
I do have some some details that that could be improved IMO:
- Camera movement feels a bit.. "off". I don't know how to describe it but it felt like it was a bit shaky or maybe too fast.
- On my 1st run I tried climbing the stairs at the bottom right of the 1st screen, but I ended up falling and returning to the title screen having to go through all the intro again, which was annoying
- Played it with a gamepad but the analog stick didn't work (this is the one I use: https://redragon.es/products/harrow-g808/)
- The falling blocks take too much time to respawn, I felt like half a minute or so
- The fire action seemed to have a tiny delay between me pressing the button and the character firing the ball. With the normal monters wasn't a big deal, but it makes the boss a bit harder.
I hope this little feedback can be of any help. I can see that you put a lot of love developing this game, and I can't wait to play the full version!